FR2465011A1 - MATERIAL CONSISTING OF A PROTECTED STEEL SHEET, METHOD FOR MANUFACTURING SAME, AND APPLICATIONS THEREOF, IN PARTICULAR TO PRESERVE BOXES - Google Patents

Abstract

THIS IS A MATERIAL IN THE FORM OF A SOFT STEEL SHEET ON AT LEAST ONE OF THE SIDES OF WHICH A TIN UNDERLAY AND A COATING CONTAINING CHROME AND CHROME OXIDE ARE DEPOSITED. ACCORDING TO THE INVENTION, THE CHROME CONTENT OF THIS COATING IS GREATER THAN 5 MGCM, AND ITS CHROME OXIDE CONTENT, EXPRESSED AS CHROME, IS LESS THAN 2.5 MGCM. APPLICATION ESPECIALLY TO PRODUCTS TO BE MADE FOR ELECTRIC WELDING, IN PARTICULAR BOX BODY FOR TIN CANS.

Description

The present invention relates generally to steel sheets

  mild type of those usually used for the manufacture of metal packaging, including

Cans.

  Among the questions raised by the manufacture of tin cans, one relates to their embodiment, 1 '

  other to the protection of the sheet which constitutes them.

  As regards their production, canned foods

  generally bear a body closed by two funds.

  Traditionally, the body is made by rolling a

  strapping, the lips of which must be joined together

  superimposed wind at the end of such rolling; the bottoms form one and the other separate parts of such a body, that it is

  necessary to then assemble to it by crimping.

  Admittedly, it is known to ensure the realization of a single holding of one of these funds with the body, by stamping and / or stretching of the whole from the same blank, but this embodiment requires a fitness for particular shaping For the assembly of the lips of a box body formed by rolling a strip, it is usually carried out by stapling, most often with counter-welding, or organic bonding, or by

electric welding.

  Compared, in particular, to stapling with counter welding,

  electric welding has the advantage of satisfying a minimum of

  consumption of sheet metal, to require no contribution from any

  this material, and to lead to minimal thicknesses favorable to a good execution of the subsequent crimping of the funds, which explains its current development; unfortunately he does

  can be implemented only if the treated sheet has a

  reduced activity, which in practice limits his field so far

  application, as specified below.

  With regard to the protection needed to avoid

  vice a corrosion of the sheet to the detriment of its qualities and / or flavor of the preserved food, it was usual until a

  recently secured it by a tinning operation, possibly

followed by a vernissage.

  As is known, the tin material obtained is usually

called tinplate.

  It consists of a mild steel sheet coated on the surface with a protective layer of tin; this one can remain free

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  or, by combination with the iron of the underlying sheet metal, be transformed into all or part of an alloy of iron and tin, by

example by reflow.

  But the current cost of tin, due in particular to the depletion of the world's resources in this metal, has now favored another method of protecting such a steel sheet, which consists in applying to it a electrolytic treatment, by passage in a chromium bath formed of a dilute aqueous solution of chromic anhydride containing, for example, acid

  sulfuric acid as catalyst, hereinafter referred to, for convenience,

chromium plating.

  Such a protection method is particularly described

  in French Patent No. 1,365,368.

  It leads to the formation of a material, commonly referred to as

  TIN FREE STEEL, (literally: tin-free steel), by abbrevia-

  TFS, consisting of a mild steel sheet coated on the surface with a protective layer containing metallic chromium and oxide

of chromium.

  In addition to the tin economy to which he leads, such

  material has various advantages, including that of a good

no adhesion of varnishes.

  But it has a major disadvantage, which is a resistance

  particularly high efficiency, making all electrical welding impractical. This is why, in food canned, its use is currently reserved for the manufacture of funds and some stamped bodies, while electric welding is practiced only with tinplate, except cases where the we

  agrees to apply an additional preparation operation

silent.

  The present invention generally aims to improve the performance of a TFS type material, in particular with regard to the resistivity, by limiting the chromium oxide content of its coating and by associating it with an underlayer

in tin.

  Of course, it has already been proposed to combine a tin-based layer and a layer based on chromium and chromium oxide, but without, in general, good weldability

  electric is more particularly sought.

  In the French patent application filed August 18, 1977 under No. 77 25340 and published under No. 2,362,056, on the one hand, the tin-based layer forming a first protective layer, is remelted and is constituted an alloy of tin and iron; and, on the other hand, the coating which surmounts this first protective layer, and which itself forms a second protective layer resulting from a chromium plating operation of the type of that explained above, consists essentially of oxide of chromium, the metal chromium content of this coating preferably being preferably zero and always less than 0.005 gk,

  0.5 μg / cm 2 of treated surface.

  If such a provision can actually lead to

  In this case I am looking for materials with good organic bonding ability and good formability, but this is not the case with electrical welding.

which is not envisaged.

  Indeed, the preferential presence of chromium oxide in the surface coating of this material leads for it to

  resistivity values that are incompatible with such electrical welding

  unless it is difficult to grind first and

Expensive coating.

  It is the same with regard to the material described in the French patent application filed August 24, 1977 under

  No 77 25886 and published under No 2,362,943, which is not

  than the fact that the tin-based undercoat

consists of free tin.

  In Japanese Patent No. 48 351136, such a free tin layer forms the surface coating of the material concerned, the chromium and chromium oxide layer then forming an underlayer underlying this coating; but in this case, the

  the tin layer can only be

  protection performance expected otherwise from

  the undercoat based on chromium and chromium oxide.

  In the French patent application filed March 31, 1978 under No. 78 09425 and published under No. 2,385,818, the first protective layer is exclusively an alloy of tin and iron, the initially deposited tin being systematically remelted to this effect, and to the underlay thus produced is applied a

  passivation coating based on chromium.

  This coating, the characteristics of which are not

  not specified, is obtained either by passing through a conventional bath

  cathodic passivation of electrolytic tinplate at

  sodium dichromate base, either by passing through a bath

  chromium-plating system of the type indicated above for

obtaining a TFS material.

  In the first case, the chromium and oxide

  The resulting chromium is so small that corrosion resistance

  result is insufficient; in the second case the

  deposit obtained must, without other substances, have a chromium oxide content

  me too important for an electric welding of the material ob-

  held can actually be considered.

  The present invention more specifically for a method for protecting a mild steel sheet leading to the production of an electrically weldable material, the material

  obtained by applying this process, and the products manufactured

  using such a material, in particular when it comes to

  products involving for their manufacture an electric welding.

  Inventioii, with regard to the protection of a

  less of the faces of a mild steel plate by a step of

  mage during which a layer of free tin is placed on said face, then a chromium plating operation during

  of which an additional layer comprising chromium metal-

  lique et chromium oxide is placed on the previous tin layer, the chromium-plating operation is carried out so that the amount of chromium metal deposited during it is greater than 5 micrograms ( pg) per cm2 of area

  of the considered face of the treated sheet.

  In practice, according to the invention, the amount of chromium deposited is between 5 and 10 μg / cm 2. Tinning can be carried out by electrolysis; it is a known technique that will not be described here in all its details. It will suffice to recall that it is usually practiced after degreasing in an alkaline medium, a rinsing with water, etching in an acidic medium and a new rinsing with water, and that it implements any electrolyte. tinning acid, as

  stannous sulphate, stannous chloride and standard fluoborate

  for example, or any alkaline tinning electrolyte, such as sodium stannate or potassium stannate by

example.

  With acidic tinning electrolyte, the conditions for

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  mage are preferably the following: - concentration of stannous ion bath: 1.5 to 40 g / i - concentration of the bath in acid, which can be sulfuric acid or 1.0 to 20 g / l phenolsulfonic acid for example: H2SO4 equivalents - concentration of the bath in products of

  addition, sulfones, diphones, or

  duit known under the name EJTSA lower than for example 30 g / i bath temperature 3000 to 600C - density of the current: 5 to 50 A / dm2

  Sheet steel, previously degreased, pickled and rinsed

  This is treated as a cathode, the anode being pure tin.

  But tinning may advantageously be achieved by etching stripping, as described in particular in the French patent

No. 1,500,185 of August 8, 1966.

  It is in this case simply preceded by an alkaline degreasing and rinsing with water, acid pickling being

  performed simultaneously with the tinning sought.

  The advantage of such etching stripping is in particular of

  to a slight deposit of free tin, particularly adherent and uniform, and well suited to obtain the desired material

according to the invention.

  The tinning pickling bath used consists, like an electrolytic tinning bath, with an aqueous solution of acid containing stannous ions, but with a lower concentration of stannous ions and a higher acid content; the sheet metal

  steel, previously degreased and rinsed, is treated in

  thode, as before, but this time the anode is

  killed by a material with high resistance to dissolu-

  anodic, graphite or ferrosilicon for example.

  The conditions of use are preferably as follows: concentration of the stannous ion bath 0.3 to 1.5 g / l - concentration of the bath in acid, which may be sulfuric acid or an acid 15 to 100 g Phenolsulfonic Equivalent - bath temperature: 20 to 8002 current density 10 to 60 A / dm 2

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  It is important in practice that the concentration of the hydrogen ion bath from the acid be sufficiently large relative to the stannous ion concentration so that the cathodic current efficiency of the deposited metal is low, less than 50%, and preferably between 5 and 25%. In all cases, electrolytic tinning or tinning etching, no remelting of the deposited tin is practiced, this tin thus remaining in the form of free tin, that is to say under

unalloyed tin form.

  The chromium in principle, can also follow a known technique: the chromium bath used is then a

  electrolytic bath of the type described in the French patent.

  No. 1,365,368 mentioned above, that is to say, an electric bath.

  trolytic material which is formed of an aqueous diketed solution of chromic anhydride, for example containing sulfuric acid as

  catalyst, and in which the worked sheet is taken as a

method.

  Those skilled in the art know, with such a bath, to control exactly

  the amount of chromium metal deposited as a function of the experimental conditions used, and in particular the current density, the amount of chromium metal deposited according to Faraday's law with a relatively poor efficiency.

but known and constant.

  It is therefore easy to ensure that the amount of chromium metal deposited is that provided according to the invention, such as

as specified above.

  In conjunction with the invention, the amount of chromium oxide used is set to be less than 2.5 μg / cm, for the desired electric welding ability, and in practice greater than 0, 6 pg / cm2, for protection also

  sought, including limits; as is customary, the quantities

  chromium oxide are expressed here, and the same is true of all that follows, in chromium, that is, in

  chromium which is oxidized in chromium oxide.

  The person skilled in the art also knows how to obtain the respect of such a quantity or content, which is the final content, that he

  conduct the chromium-plating operation, that is, that

  leading to a higher content than that sought, it implements a correction process of dissolution, by more or less prolonged stay of the treated sheet in the chromium plating bath without passing current, able to reduce this content to the desired value . 2e1i65041 'According to the invention, it is preferable, however, to implement a special chromium bath, making it easier to obtain

  the low chromium oxide content sought.

  This bath, which may also contain sulfuric acid as a catalyst, and in which the worked sheet is treated as previously in cathode, is, as previously, based on chromic anhydride, but is buffered from the beginning by adding

  dichromate, and for example sodium dichromate.

  It is therefore typically a mixture of

  pon of chronic anhydride and dichromate.

  The dichromate stamping thus provided by the invention for

  the chromium bath used has the advantage of compensating

  the hydrogen loss due to the ionic decomposition of chromic anhydride, to limit the rise of pH in

  the diffusion layer on the cathode constituted by the

  valiant, and hence to minimize the training on it of

chromium oxide.

  In practice, the pH of the buffered chromium plating according to the invention is between 0.7 and 1.5, and is preferably

neighbor of 1.

  The buffered chromium bath is characterized in particular by its total content on chromium t; it is also characterized by the ratio r of the respective concentrations between on the one hand the

  chromium which is present in the form of bichromic ions,

  bichromate, and on the other hand the chromium present therein in the form of chronic acid, derived from the chronic anhydride, namely: H CrO4- 1 | Preferably, according to the invention, the total chromium content of the chromium plating bath used, expressed in mole fractions, is between 0.50 and 0.71, and, expressed in g / l, and 37, limits included; and, jointly, the report

  r defined above is between 0.5 and 2 inclusive.

  Table I below gives, as a function of the values of t, in mole fractions, on the one hand, and r, on the other hand, the values x and y, in mole fractions, of the anhydride content.

  chromic and bichromic ions, respectively, of the chromium bath.

buffered mage according to the invention:

TABEDAU I

  In short, in molar fractions, the chromic anhydride content of the buffered electrolytic chromium plating bath according to the invention is between 0.17 and 0.47, and its ion content

  bichromatic between 0.08 and 0.24, limits included.

  If, for example, the dichromate used is bichromate

  the values of Table I become, in grams / liter, taking into account the corresponding molar masses, those of Table II below:

TABELAU II

  in grams / liter sodium, following t 0.5 I 2 x = 33.0 x = 25.0 x = 17.0 26 y = 21.0 y = 32.8 = 44.5 x = 40.0x = 30 , 0 = 20.0 31.2 y = 26.2 y = 39.3 y = 52.4 x = 47.0 x = 36.0 x = 24.0 37 y = 31.4 y = 47.2 y = 62.9 In short, in the case of the use of dichromate, in grams / liter, of the buffered chromium plating bath

  the invention is between 17.0 and 47.0 for the anShy-

  t 0.5 1 2 x = 0.33 x = 0.25 x = 0.17 0.50 y = 0.08 y = 0.125 y = 0.17 x = 0, 40 x = 0.30 x = 0 , 0.60 y = - = 0, s15 y = 0.20 x = 0.47 x = 0.36 x = 0.24 0.71 y = 0.12 y = 0.18 y = 0.24

9'Z! 6504 2

  chromic druid, and 21.0 and 62.9 for sodium dichromate,

limits included.

  But other dichromates can obviously be put in

  and especially potassium dichromate.

  The corresponding contents are deduced as previously

  those of Table I, taking into account molar masses.

  In all cases, in order to initiate the deposition of chromium, the buffered chromium plating bath according to the invention also comprises

  preferably, from the beginning, trivalent chromium ions.

  These can for example be obtained in situ by reacting hydrogen peroxide with chromic anhydride surplused for this purpose; this additional content of chromic anhydride is added in this case to that corresponding to the values

  sought for total chromium content t and ratio r.

  In all cases also, it is sought to minimize the chloride content of the chromium plating bath, in order to prevent these from promoting the deposition of excessive amounts of oxide.

of chromium.

  In any case finally, the operation of the chromium applied to the worked sheet is, as is usual, followed by rinsing, drying, and surface lubrication, for example at

  dioctyl sebacate or with cottonseed oil.

  Anyway, thanks to the invention, it is obtained a material, comprising a mild steel sheet, resulting for example

  cold rolling, on at least one side of which is

  applied a first protective layer formed of free tin, and secondly a second protective layer, which contains chromium metal and chromium oxide, and whose content of metallic chrome is greater than

5 μg / cm of surface.

  This chromium content is in practice between 5 and

pg / cm2.

  At the same time, the chromium oxide content of the second protective layer, expressed as chromium, as specified above, is, according to the invention, less than 2.5 sg / cm 2 and greater than

0.6 μg / cm 2, limits included.

  On the other hand, the free tin content of the first protective layer can be reduced, and for example be lower

at 0.5 gram per m2 of area.

  Nothing in fact prevents this content of free tin, which is easy to control for those skilled in the art, to be

24 65011

  higher. But, the invention is satisfied with a reduced content, this

  which, given the price of tin, is particularly

  geous. In practice this content of free tin is according to the invention preferably close to 0.25 + 0.05 g / m 2. It goes without saying, however, that we would not go outside the scope of

  the present invention with a higher free tin content.

  In any event, it appears that the presence thus

  Jacente free tin according to the invention, before the chromium plating operation, is favorable to a reduction of the chromium oxide set

  in place during this chromium plating operation.

  The presence of underlying tin is thus doubly favorable

  to obtain a good aptitude for electric welding, since, to its own effects, are added those of the limitation

  of the chromium oxide content of which it is at the origin.

  In fact, and the tests confirm, the following material

  the invention is characterized in particular by an electrical resistivity

  that low enough to allow for electric welding

under normal conditions.

  Comparative tests were made between, on the one hand, samples (A) of materials according to the invention, and samples (B) of materials, type TFS, obtained under the same experimental conditions with respect to chrome plating operation, but without prior placement of the tin undercoat.

  These tests consisted of an electrical resistance measurement

  250C, performed on materials that have undergone a heat treatment of 30 minutes at 2000C, and made with two copper electrodes applied respectively to the

  two sides of the treated sample, along a surface of

  10 mm tact, the treated sample being set up in a single thickness. They consisted in measuring the voltage drop between these electrodes, when a 1 ampere continuous current flows between them, and that a contact pressure of

the order of 1400 bars.

  For samples (A), resistances lower than

x 10-5 ohms were recorded.

  With the samples (B), these resistances have always been

greater than 2000 x 10-5 ohms.

  However, under normal conditions, electrical welding is impossible when the resistance is greater than 1000 x 10-5 ohms. Practical experience confirms that the material according to the invention is electrically weldable, whereas a prior art TFS material is not electrically weldable due to its resistivity

excessive electric.

  In such an aptitude for electric welding, the presence

  free undercoat is not the only one to intervene.

  Indeed, experience shows that such an electrical welding remains possible when all or part of this free tin has alloyed with iron by diffusion, as is the case when, as provided in the above tests, the treated material is the object of heating at a temperature below the temperature of

melting tin.

  It may be thought that, with respect to the aptitude to

  still observed, the process of diffusion formation of the iron-tin alloy obtained is not indifferent, nor

  the fact that this alloy was formed only after deposit of the

  protective guard containing chromium and chromium oxide.

  Anyway, it is possible, using a material according to the invention, to obtain products, can body for cans for example, having two lips secured directly to each other. other by electric welding, without

  material, nor any previous grinding, nor any other interventions

  ticular. As non-limiting examples, we will give below

  various examples of processes leading to a tinned material followed

the invention

EXEIIPLE I

  A cold-reduced mild steel sheet having a thickness of 0.921 mm is electrolytically degreased in a solution of sodium carbonate at 10 g / l and the sheet is then etched in dilute sulfuric acid.

  After rinsing with water, the sheet is subjected to

  electrolytic tinning under the following conditions Composition of the electrolysis bath: - Stannous sulphate 83.3 g / l - Di-Ammonium hydrogen carbonate: 66.6 g / l - Citric acid 166.6 g / l

12 ú4650 1

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  - 20% ammonium hydroxide: 153.3 g / 1 - Wetting agent 0.15 g / l Electrolysis bath temperature: 45 C Cathodic current density 4A / dm2 Under these conditions, the weight of the coating free tin

deposited is 0.25 g / m2.

  After rinsing with water and drying, the sheet thus covered with a layer of free tin is subjected to a chromium plating operation

  cathodic electrolyte, under the following conditions:

  : Composition of the electrolysis bath: - Chromic anhydride: 30.0 g / 1 - Potassium dichromate: 44.1 g / 1 Sulfuric acid: 0.49 g / l - Chromium ions (obtained in situ by reaction of 8 , 10 g / l of hydrogen peroxide on 5.08 g / l of chromic anhydride): 2.64 g / l Temperature of the electrolysis bath: 50 C Cathodic current density 55 A / dm2 Under these conditions the chromium and chromium oxide deposited layer contains 9.2 μg / cm 2 of metallic chromium and

2.1 Pg / cm of chromium oxide.

  After chromium plating, the treated sheet is rinsed with water, dried,

  and coated with a thin film of dioctyl sebacate.

  The electrical resistance of the material obtained is, under the experimental conditions specified above, 50 × 10 -5 ohms.

* EXAMPLE II

  The experimental conditions are the same as those of Example I, with the exception of those relating to the chromium plating operation, which are as follows: Composition of the electrolysis bath: Chromic anhydride: 60.0 g / l - sulfuric acid: 0.49 g / l - chromium ions (obtained as above): 2.64 g / l Temperature of the electrolysis bath: 50 C Cathodic current density: 48 A / dm Under these conditions, the The chromium metal content of the chromium and chromium oxide-deposited layer is 5.6 13 "-1 650 cm -1 per room of the sheet in the electrolysis bath without current flow, the oxide content of this layer

  is, by dissolution, reduced to 2.2} g / cm 2.

  The electrical resistance of the obtained material is 55 x 10-5 ohms.

EXAMPLE III

  The experimental conditions are the same as those

  of Example II, with the following modifications.

  With regard to the tinning operation, the cathodic current density used is 6 A / dm2. In these circumstances, the

  The weight of the free tin coating deposited is 1.05 g / m.

  As regards the chromium plating operation, the experimental conditions are as follows: Composition of the electrolysis bath: - Chromic anhydride: 4 g / l - Sulfuric acid: 0.35 g / l - Chromium ions (obtained in by reaction of 0.61 g / l of hydrogen peroxide with 0.38 g / l of chromic anhydride): 0.2 g / l Temperature of the electrolysis bath: 520 C Cathode current density: 48 A In these conditions, the chromium metal content of the layer based on chromium metal and chromium oxide deposited is 7.4 μg / cm 2; and, by dissolution, as before, the

  Chromium oxide content is reduced to 1.6 μg / cm 2. The electrical resistance of the obtained material is 9 x 10-5 ohms.

EXAMPLE IV

  The conditions of experimentation are the same as those of Example I, except for those of the tinning operation, which are as follows: Composition of the electrolysis bath: stannous sulphate: 63.3 g / l - Paraphenolsulfonic acid: 15 g / l equivalent H2SO4 - Sulfones: 6 g / l Wetting agent: 0.1 g / l

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  Temperature of the electrolysis bath: 400C Cathodic current density 8 A / dm2 Under these conditions, the weight of the free tin coating

deposited is 0.25 g / m2.

  The chromium metal content of the chromium-based layer

  me and chromium oxide deposited during the chro-

  The next mage is 7.1 μg / cm and its chromium oxide content is 1.9 μg / cm. The electrical resistance of the obtained material is 45 ×

105 ohms.

EXAMPLE V

  The experimental conditions are the same as those of Example I, except for those of the tinning operation, which is provided by electrolytic etching electroplating, under the following conditions, so that the treated sheet It is previously subject only to degreasing followed by rinsing: Composition of the electrolytic bath: - Stannous sulphate 1.2 g / l - Acid, for example sulfuric acid 100 g / l Bath temperature electrolyte: 35-C cathodic current density 15 A / dm Under these conditions, the weight of the deposited free tin coating is 0.3 g / m

  The chromium metal content of the chromium-based layer

  me and chromium oxide deposited during the chro-

  Next mage is 7.0 μg / cm 2 and its chromium oxide content of 2 of 1.9 μg / cm. The electrical resistance of the product obtained is 45 x 10-5 ohms. It can be seen from the previous examples that the resistance

  electrical connection of the material according to the invention is in practice always

  days less than 100 x 10-5 ohms.

  It is stated in this regard that, in the foregoing, the

  measurements of electrical resistance were, as mentioned above,

  above, made after submitting the materials concerned to

  a heat treatment of 30 minutes at 2000C, simulating the

  age involved in the possible application of an organic coating.

  Before this treatment, the electrical resistance of these

  terials is much weaker; she's on the order of

x 10-5 ohms.

  In addition, it must be clearly understood that the above is a laboratory process with dead treatment baths and experimental adaptations which fall within the competence of the man in charge. art, will in practice inevitably be necessary to move to industrial processes,

  with online treatment baths.

  In particular, in the preceding examples, it has not been indicated the amounts of current required, or, what is equivalent, the necessary processing times. However, those skilled in the art will know from the data indicated for dead baths (bath composition, temperature, current density), to determine under industrial conditions the quantities of current, or the treatment times, allowing to obtain the

expected results.

  Obviously, therefore, one would not escape the invention by

  specifying one or other of the experimental conditions specified above.

  The present invention is not limited to the examples

  implementation plans thus clarified, but extends to all

execution variant.

16 2I65 011

Claims (16)

  1. A method for protecting at least one face of a mild steel sheet of the kind comprising a tinning operation, during which a layer of free tin is placed on said face, and then an operation in which an additional layer comprising metallic chromium and chromium oxide is placed on the preceding tin layer, characterized in that the chromium plating operation   is conducted in such a way that the amount of metallic chromium   that deposited during the latter is greater than at least 5 micrograms per cm2 (ig / cm) of surface of the face considered sheet metal worked.   2. Method according to claim 1, characterized in that the chromium plating operation is carried out so that the amount of chromium metal deposited is between 5 and 10 Pg / cm.   3. Process according to any one of claims 1,   2, characterized in that the amount of chromium oxide set up, expressed in chromium, is higher than   at 0.6 μg / cm 2 and less than 2.5 μg / cm 2, limits included.   4. Process according to any one of claims 1 to   3, and according to which the chromium plating operation comprises the passage   worked sheet, taken as cathode, in an electro-bath   lytic composition comprising a dilute aqueous solution of chromic anhydride   characterized in that, by the residence of the sheet worked in said electrolytic bath without passage of current, after the chromium plating operation, the chromium oxide content of the chromium-based layer is brought back to the desired value. oxide of   chromium deposited during this chromium plating operation.   5. Process according to any one of claims 1 to   3, and according to which the chromium plating operation comprises the passage   worked sheet, taken as cathode, in an electro-bath   lytic composition comprising a dilute aqueous solution of chromic anhydride   characterized in that from the beginning the electrolytic bath is stamped with a dichromate.   6. Process according to Claim 5, characterized in that the total chromium content of the electrolytic bath is included   between 26 g / l and 37 g / l, limits included.   7. Process according to Claim 5, characterized in that the ratio of the respective concentrations in the bath 24 65011   the chromium present therein in the form of dichromic ions, originating from dichromate, and on the other hand the chromium which is present therein in the form of chromic acid, originating from chromic anhydride, is between 0.5 and 2, limits included. 8. Process according to claim 5, characterized in that, in mole fractions, the chromic anhydride content of the electrolytic bath is between 0.17 and 0.47, and its content   in bichromic ions between 0.08 and 0.24, limits included.   9. Process according to claim 5, characterized in that   that the pH of the electrolytic bath is between 0.7 and 1.3.   10. Material of the kind comprising a mild steel sheet on at least one of the faces of which are applied, firstly, a first protective layer formed of tin, and secondly, a second layer of protection, containing metallic chromium and chromium oxide, characterized in that,   according to the method according to any one of the   1 to 9, the metallic chromium content of its second protective layer is greater than at least 5 micrograms per cm (ig / cm) of surface.   11. Material according to claim 5, characterized in that   the chromium oxide content of its second layer of   chromium is less than 2.5 pg / cm2 and above   less than 0.6 pg / cm 2, limits included.   12. Material according to any one of claims 10,   11, characterized in that the tin content of its first cou-   protection is less than 0.5 gram per m (g / m 2) area. 13. Material according to claim 12, characterized in that the tin content of its first protective layer is close to 0.25 + 0.05 g / m2.   14. Material according to claim 13, characterized in that its electrical resistivity is sufficiently low to allow electric welding.   15. Material according to claim 14, characterized in that, after a heat treatment of 30 minutes at 200 ° C, its   electrical resistance measured at a single thickness, between two electri-   copper trodes each respectively applied on its two faces along a contact surface of 10 mm and a pressure of   contact of the order of 1400 bars, under a current of 1 ampere con- 18 2465011   tinu, is less than 100 x 10 -5 ohms.   16. Product, in particular box body, of the kind formed from a material of which two lips are joined to one another by electric welding, characterized in that said   material is in accordance with any one of claims 10 to 15.